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Ammonia is a key alkaline species, playing an important role by neutralizing atmospheric acidity and inorganic secondary aerosol production. On the other hand, the NH3/NH4 (+) increases the acidity and eutrophication in natural ecosystems, being NH3 classified as toxic atmospheric pollutant. The present study aims to give a better comprehension of the nitrogen content species distribution in fine and coarse particulate matter (PM2.5 and PM2.5-10) and to quantify ammonia vehicular emissions from an urban vehicular tunnel experiment in the metropolitan area of São Paulo (MASP). MASP is the largest megacity in South America, with over 20 million inhabitants spread over 2000 km(2) of urbanized area, which faces serious environmental problems. The PM2.5 and PM2.5-10 median mass concentrations were 44.5 and 66.6 μg m(-3), respectively, during weekdays. In the PM2.5, sulfate showed the highest concentration, 3.27 ± 1.76 μg m(-3), followed by ammonium, 1.14 ± 0.71 μg m(-3), and nitrate, 0.80 ± 0.52 μg m(-3). Likewise, the dominance (30 % of total PM2.5) of solid species, mainly the ammonium salts, NH4HSO4, (NH4)2SO4, and NH4NO3, resulted from simulation of inorganic species. The ISORROPIA simulation was relevant to show the importance of environment conditions for the ammonium phase distribution (solid/aqueous), which was solely aqueous at outside and almost entirely solid at inside tunnel. Regarding gaseous ammonia concentrations, the value measured inside the tunnel (46.5 ± 17.5 μg m(-3)) was 3-fold higher than that outside (15.2 ± 11.3 μg m(-3)). The NH3 vehicular emission factor (EF) estimated by carbon balance for urban tunnel was 44 ± 22 mg km(-1). From this EF value and considering the MASP traffic characteristics, it was possible to estimate more than 7 Gg NH3 year(-1) emissions that along with NOx are likely to cause rather serious problems to natural ecosystems in the region.